The objective of this application is to develop a general-purpose computer simulator for high intensity focused ultrasound (HIFU) in therapeutic application. A novel computational model is proposed to significantly improve the current modeling capability of therapeutic ultrasound. It will provide the first commercial HIFU simulator to enable the simulation and planning of therapeutic ultrasound protocols. The proposed method is based on the spectral element time-domain (SETD) method recently developed by the research team for linear wave propagation. As in linear wave propagation, it is expected that the SETD method for nonlinear HIFU simulation is also significantly more accurate and more efficient than the conventional finite-difference time-domain (FDTD) method currently being used in research laboratories. In Phase I of this SBIR project, the SETD method will be developed and demonstrated for a quasi-3D HIFU simulator, where the nonlinear acoustic pressure field is assumed 2D axisymmetric, while the temperature field is 3D. The aims in Phase I of this application are to (a) develop the mathematical algorithm of a quasi-3D SETD method for the simulation of high-intensity ultrasound wave interacting with heterogeneous tissue; (b) implement the SETD method for arbitrary quasi-3D heterogeneous media; and (c) perform extensive validation and evaluation of the HIFU simulator with other reference methods. The aims in a follow-up Phase II project are to (a) develop a full 3D SETD algorithm for heterogeneous media; (b) implement the 3D SETD method for realistic therapeutic environment; (c) develop a user-friendly graphic interface environment for practitioners to use the HIFU simulator for design optimization of ultrasound therapeutic protocols; and (d) commercialize this novel therapeutic ultrasound computational tool. This application will develop a general-purpose computer simulator for high intensity focused ultrasound (HIFU) therapeutic planning. Such a computational tool is highly valuable for doctors to plan an HIFU surgery as it gives accurate prediction of what will happen during the surgery by simulating high intensity ultrasound propagation in tissue. [unreadable] [unreadable] [unreadable]